use proc_macro2::TokenStream;
use quote::quote;
use syn::{parse_macro_input, Data, DeriveInput, Fields::Unnamed, Ident, Variant};

/// Generates implementation ActivityHandler for an enum, which looks like the following (handling
/// all enum variants).
///
/// Based on this code:
/// ```ignore
/// #[derive(serde::Deserialize, serde::Serialize)]
/// #[serde(untagged)]
/// #[activity_handler(LemmyContext, LemmyError)]
/// pub enum PersonInboxActivities {
///  CreateNote(CreateNote),
///  UpdateNote(UpdateNote),
/// }
/// ```
/// It will generate this:
/// ```ignore
/// impl ActivityHandler for PersonInboxActivities {
///     type DataType = LemmyContext;
///     type Error = LemmyError;
///
///     async fn verify(
///     &self,
///     data: &Self::DataType,
///     request_counter: &mut i32,
///   ) -> Result<(), Self::Error> {
///     match self {
///       PersonInboxActivities::CreateNote(a) => a.verify(data, request_counter).await,
///       PersonInboxActivities::UpdateNote(a) => a.verify(context, request_counter).await,
///     }
///   }
///
///   async fn receive(
///   &self,
///   data: &Self::DataType,
///   request_counter: &mut i32,
/// ) -> Result<(), Self::Error> {
///     match self {
///       PersonInboxActivities::CreateNote(a) => a.receive(data, request_counter).await,
///       PersonInboxActivities::UpdateNote(a) => a.receive(data, request_counter).await,
///     }
///   }
/// ```
#[proc_macro_attribute]
pub fn activity_handler(
    attr: proc_macro::TokenStream,
    input: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let derive_input = parse_macro_input!(input as DeriveInput);
    let derive_input2 = derive_input.clone();
    let attr = proc_macro2::TokenStream::from(attr);
    let mut attr = attr.into_iter();
    let data_type = attr.next().expect("data type input");
    let _delimiter = attr.next();
    let error = attr.next().expect("error type input");

    let enum_name = derive_input2.ident;

    let (impl_generics, ty_generics, where_clause) = derive_input2.generics.split_for_impl();

    let enum_variants = if let Data::Enum(d) = derive_input2.data {
        d.variants
    } else {
        unimplemented!()
    };

    let impl_id = enum_variants
        .iter()
        .map(|v| generate_match_arm(&enum_name, v, &quote! {a.id()}));
    let impl_actor = enum_variants
        .iter()
        .map(|v| generate_match_arm(&enum_name, v, &quote! {a.actor()}));
    let body_verify = quote! {a.verify(context, request_counter).await};
    let impl_verify = enum_variants
        .iter()
        .map(|v| generate_match_arm(&enum_name, v, &body_verify));
    let body_receive = quote! {a.receive(context, request_counter).await};
    let impl_receive = enum_variants
        .iter()
        .map(|v| generate_match_arm(&enum_name, v, &body_receive));

    let expanded = quote! {
        #derive_input
        #[async_trait::async_trait(?Send)]
        impl #impl_generics activitypub_federation::traits::ActivityHandler for #enum_name #ty_generics #where_clause {
          type DataType = #data_type;
          type Error = #error;
            fn id(
                &self,
              ) -> &Url {
              match self {
                #(#impl_id)*
              }
            }
            fn actor(
              &self,
            ) -> &Url {
              match self {
                #(#impl_actor)*
              }
            }
            async fn verify(
                &self,
                context: &activitypub_federation::data::Data<Self::DataType>,
                request_counter: &mut i32,
              ) -> Result<(), Self::Error> {
              match self {
                #(#impl_verify)*
              }
            }
            async fn receive(
              self,
              context: &activitypub_federation::data::Data<Self::DataType>,
              request_counter: &mut i32,
            ) -> Result<(), Self::Error> {
              match self {
                #(#impl_receive)*
              }
            }
        }
    };
    expanded.into()
}

fn generate_match_arm(enum_name: &Ident, variant: &Variant, body: &TokenStream) -> TokenStream {
    let id = &variant.ident;
    match &variant.fields {
        Unnamed(_) => {
            quote! {
              #enum_name::#id(a) => #body,
            }
        }
        _ => unimplemented!(),
    }
}